Many communication networks that provide high bit-rate transport over a shared medium are characterized by non-continuous or burst data transmission. A typical PON includes a plurality of optical network units (ONUs) connected to an optical line terminal (OLT) via a passive optical splitter. Traffic data transmission is performed over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, the OLT continuously transmits downstream data to the ONUs and receives upstream burst data sent to the OLT from ONUs.
An ONU transmits data to the OLT during different time slots allocated by the OLT. Transmission from an ONU to the OLT is in the form of a burst. An ONU includes an optical transceiver that receives continuous data and transmits burst data. Every burst data transmission is preceded by a Start-of-Burst (SoB) signal and followed by an End-of-Burst (EoB) signal that respectively enables and disables the optical transmitter. The OLT must identify these signals to properly recover the transmitted data. Thus, no other signals should be output when the ONU transceiver does not transmit.
To allow this, PON communication standards such as the Gigabit PON (GPON) requires that a low-logic value (‘0’) signal will be transmitted when there is no data to output. For example, as shown in FIG. 1, from T0 to T2 a low-logic value signal is transmitted and at T1 the SoB/EoB signal is asserted. As illustrated in FIG. 1, the SoB/EoB signal is asserted prior to the transmission of the data to enable the laser of the optical transmitter to reach its working point. During the time interval from T2 to T3, a burst data signal is transmitted, and thereafter a low-logic value signal is transmitted. At T4, the SoB/EoB signal is de-asserted to disable the optical transmitter.
FIG. 2 shows a schematic diagram of an ONU 200 that includes a medium access control (MAC) module 210 that generates the burst data and low-logic value signals. The MAC module 210 is a logic component implemented as an integrated circuit (IC). The ONU also includes an optical transceiver 220 which its transmitter part generates and transmits optical signals respective of the input data signals provided by the MAC module 210.
The MAC module 210 and optical transceiver 220 operate at different direct current (DC) levels V1 and V2 respectively. Typically, the DC level of the MAC module 210 (V1) is significantly lower than the DC level of the optical transceiver 220 (V2), in particular, when the size of the IC including the MAC module 210 is designed to support advanced semiconductor fabrication techniques.
Typically, burst data signals generated by the MAC module 210 are offset by a certain biased DC level. To remove the biased DC level of a burst data signal, which is an alternating current (AC) signal, an AC coupling circuit 230 is utilized to interface between the MAC module 210 and the optical transceiver 220. The AC coupling circuit 230 is comprised of serial capacitors and resistors connected in the data path between the MAC module 210 and the optical transceiver 220. The AC coupling is required to filter and block DC and low frequency signals. However, during AC coupling, use of coupling capacitors may cause base line wander problems to occur when a long string of information is repeatedly included in a sequence of identical bits. For example, in high-speed communication standards, such as GPON, the number of identical bits is typically large, thus transmitting the pattern depicted in FIG. 1 would results in losing information, in practical operation, when, the laser of the optical transceiver is turned on or shut down.
Some techniques discussed in the related art suggest decoding the transmitted data using DC balanced codes, such as a Manchester code, an ANSI Fiber-Channel 8B10B code, and the like. However, advanced high-speed communication standards, such as the GPON and 10XPON do not allow decoding the transmitted data, but rather require that the transmitted burst data signals will be scrambled using a polynomial method. In addition, such techniques result in loss of data during the beginning of the burst.
Therefore, it would be advantageous to provide a solution that limits the disadvantages of existing AC coupling techniques.